There are many situations in which electromagnetic energy is to be distributed into an evenly distributed ring shaped output requirement. In the vast majority of these situations, a high efficiency transfer of source energy is desirable. This is particularly true in regulated lighting. For example overland vehicle safety lighting, aircraft lighting, and marine lighting are examples that require specific light distribution patterns that are generally mandated by government regulations to have minimum and maximum illumination values. In most cases, a minimum photometric or radiometric output must be met by the illumination device. In many safety lighting cases, the output distribution requirements consist of an intensity distribution revolved around a vertical lighting axis. The typical requirements have a 360 degree ring shaped or a less than 360 degree partial ring distribution having the characteristic that for any cone projecting from the source intersecting the requirement results in a constant intensity distribution throughout the conical section.
For example, a 2 mile rated stem light requires an even minimum intensity of 4.2 Candellas (Cd) for 360 degrees in the horizontal plane and for +/−5 degrees in an orthogonal vertical plane as measured by a type A goniometer. Lamps of this type are typically mounted on a pole with an incandescent source, the source filament is oriented vertically and the light projecting from the sides of the filament is concentrated in a vertical direction and is emitted in a 360 degree circle around the vertical axis of the lamp. In another example the light energy of the previous example is attenuated by an opaque light shield creating a 135 degree section of a ring in the horizontal direction.
Light Emitting Diodes (LEDs) are solid state electrical devices with high efficiencies and long lives as compared to other light sources. LEDs are generally impact resistant, use very little power and often have 100,000 hour life spans. These features make these devices preferable for use in safety lighting. The primary disadvantage of LED light sources however is their cost. If the efficiency of an optical device to distribute light from the LED into the required or regulated pattern is improved, fewer LEDs can be used resulting in more cost accessible interior illumination and safety lighting devices.
It is also worth noting that in the case of LED devices, the diode chip which provides the illumination must be kept to a minimum temperature. Higher LED temperature results in reduced product life and can change the output color and intensity of the LED. Thus, there remains a need for a cost-affordable lamp using LEDs to provide a substantially even intensity ring shaped output distribution.
Recently, LED manufacturers have turned to surface mountable LED devices that have superior heat removal from the diode junction and higher optical flux per watt. These devices are now being regularly provided with a flat output surface free from the source distorting optics of past LEDs. These devices typically have very wide output distributions with typical viewing angles greater than 100 degrees. The viewing angle is typically defined as the full angular width of the optical distribution where the light output reaches 50% of the intensity measured on the optical axis. LEDs of this type have generally symmetrical outputs around the center or optical axis. Thus, a device having a viewing angle of 10 degrees describes a conical output distribution where 50% of the peak intensity value occurs at 5 degrees from the optical or center axis of the device. A 120 degree viewing angle device, which is a very common wide output angle LED, defines a device which has an output intensity of 50% at an angle of 60 degrees from the optical axis. These LED's have output intensity distributions which closely follow a lambertian plane source emitter and emit light in a 180 degree hemisphere.
The increased availability of high output LEDs with hemispherical output and intensity closely following that of a Lambertian plane emitter has provided a unique opportunity for the development of new optical lens shapes for meeting government requirements. These LEDs output a highly diffused illumination pattern with a very predictable intensity distribution closely following the trigonometric cosine function.
In order to efficiently meet ring shaped light requirements for a marine navigation light application using a hemispherical emitting LED, the energy must be collected, concentrated and directed to the side and below the plane of the source. In order to redirect the light energy greater than 30 degrees form its emission direction it is advantageous to use reflection to change the light direction. Reflective surfaces can be created using metallization, dielectric coatings or by total internal reflection inside a transparent material. In production dielectric coatings are often too expensive and are difficult to create on a curved surface. Metallic coating type reflectors typically have light absorption levels of 20% or more. This makes it more desirable to use internal reflection whenever possible.
Internal reflection occurs when electromagnetic energy traveling through a transparent material strikes an outer surface at an angle to the surface normal greater than the critical angle for the material. One hundred percent of the light energy is reflected back into the lens material on a path according to the laws of reflection.
The ideal shape of a reflector for an expanding source to be redirected into a ring shaped pattern is an inverted cone. The sides of the cone are ideally non-linear and are shaped in a manner which allows the light energy to be concentrated and substantially directed toward the final output ring requirement.
In many cases, internal reflection results in thick cross-section lens material and long beam paths inside the material. Thick materials are inherently difficult to mold as most materials shrink when cooling creating internal stresses and surface deformations in the final part. Also, these thick materials often have long beam paths resulting in a need for high clarity materials to minimize beam attenuation.